1. Field of the Invention
The present invention relates to creating planar surfaces. More particularly, the present invention relates to chemical mechanical polishing apparatuses and methods employing ultrasonic processing of the polishing slurries to enhance the planarization of semiconductor substrate wafer surfaces.
2. Description of the Invention Background
Integrated circuits are typically constructed by depositing layers of predetermined materials to form circuit components on a wafer shaped semiconductor substrate. The formation of the circuit components in each layer generally produces a rough, or nonplanar, topography on the surface of the wafer. The resulting nonplanar surface must be made smooth, or planarized, to ensure a proper surface for the formation of subsequent layers of the integrated circuitry by eliminating defects in the surface that can result in flawed or improperly performing circuitry.
Planarization of the outermost surface of the wafer is performed in two ways, locally over small regions of the wafers and globally over the entire surface. Typically, a layer of oxide then may be deposited over the exposed circuit layer to provide an insulating layer for the circuit and to locally planarize regions by providing a continuous layer of material. A second layer of material is then deposited on top of the insulating layer to provide a surface that can be globally planarized without damaging the underlying circuitry. The second layer is generally composed of either an oxide or a polymer. Thick oxide layers can be deposited using conventional deposition techniques. Spin coating is a commonly used technique to form the thick polymer layers on a wafer. While those techniques are useful in producing continuous uniform thickness layers, neither technique is particularly effective at producing a globally planar surface when applied to a nonplanar surface. As such, additional surface preparation is generally required prior to forming additional circuit layers on the wafer.
Methods for globally planarizing the outermost surface of the wafer include chemical etching, press planarization and chemical mechanical polishing (CMP), also referred to as chemical mechanical planarization. In chemical etching, the second layer is deposited over the preceding layers as described above and is chemically etched back to planarize the surface. The chemical etching technique is iterative in that following the etching step, if the surface was not sufficiently smooth, a new layer of polymer or oxide must be formed and subsequently etched back. This process is time consuming, lacks predictably due to iterative procedure for obtaining a planarized surface and consumes significant amounts of oxides and/or polymers in the process. In global press planarization, the second layer is planarized through the application of planar force against a planar surface that is sufficient to deform the surface of the second layer to assume a planar topography. A possible limitation to this technique is that a deformable material must be used to form the second layer.
In the CMP technique, a chemically reactive polishing slurry is used in conjunction with a polishing pad to provide a synergistic combination of chemical reactions and mechanical abrasion to planarize the surface of the second layer on the wafer. The polishing slurries used in the process are generally composed of an aqueous basic or acidic solution, such as aqueous potassium hydroxide (KOH), containing dispersed particles, such as silica or alumina. The polishing pad are typically composed of porous or fibrous materials, such as polyurethanes, that provide a somewhat compliant surface in comparison to the wafer. The polishing takes place by moving the polishing pad and/or the wafer and contacting the pad and the wafer in the presence of the polishing slurry. The wafer is polished for a period of time sufficient to achieve a desired surface finish on the layer. If the wafer is not polished for a sufficient length of time, the desired finish will not be achieved. On the other hand, if the wafer is polished for a period of time longer than necessary, the continued polishing may begin to deteriorate the surface finish. The ability to control the time required to polish the surface of the wafer can greatly improve productivity by allowing for the automation of the process, increasing the yield of properly performing wafers and a reducing the number of quality control inspections necessary to maintain the process.
A delicate balance exists in the formulations used in the CMP techniques to achieve the desired polishing effect and deviations therefrom will result in undesirable variations in the surface quality. For example, if the chemical concentration is too low, the desired chemical reactions may not proceed at an appreciable enough rate to achieve the desired polishing effect, while if the chemical concentration is too high, etching of the surface may occur. Likewise, if the particulate concentration is too low or the particle size too small, mechanical polishing will not proceed at a sufficient rate to achieve the desired polishing effect in the time provided, while if the particulate concentration is too high or the particles are too large, then the particulates will undesirably scratch the surface, instead of polishing it. The scratches that remain following polishing are often a source of variability in the performance of the finished integrated circuit resulting in problems, such as uneven interconnect metallization across a planarized surface or contamination effects due to the presence of voids or particles in the scratches.
Chemical mechanical polishing currently suffers from certain inherent problems. A first problem is that the chemicals that are needed to perform the process are relatively expensive and are generally not recyclable. It is therefore desirable to minimize the amount of chemicals used in the process to not only reduce the up front costs of purchasing and storing the chemicals, but also back end costs of waste disposal. In addition, the technique is relatively slow and time consuming. These problems are difficult to overcome without upsetting the chemical mechanical balance necessary to bring about the desired polishing effect.
In addition, CMP techniques, often experience significant performance variations over time that further complicate the automated processing of the wafers. The degradation in performance is generally attributed to the changing characteristics of the polishing pad during processing. The changes in the polishing pad result from particulates removed form the substrate during the polishing process, as well as from the slurry becoming lodged in or hardening on the surface of the pad, thereby changing the surface roughness of the pad, which is intimately associated with mechanical polishing effectiveness. Also, chemicals may become unevenly distributed in the pad resulting in variations in the chemical polishing rate. Both of these problems tend to increase the variability of the process.
Many of the prior art efforts to decrease the variability of the CMP technique have generally been directed toward overcoming the problems associated with the variability of the polishing pad. U.S. Pat. No. 5,522,965 to Chisholm et al., U.S. Pat. No. 5,399,234 to Yu, and U.S. Pat. No. 5,245,790 to Jerbic all disclose methods to improve the mechanical effectiveness and the repeatability of the polishing technique by acoustically agitating the pad-slurry-wafer interface using ultrasound to prevent or slow the accumulation of particulate matter in and on the pad. Through the use of those methods, it is hoped that the polishing pad will not have to be removed and reconditioned as often, providing not only reduced variability but increased productivity as a result of less downtime. However, a problem with these techniques are that the control over the character of the particles in the slurry occurs after the slurry has contacted and possibly scratched the surface of the wafer. While these techniques may decrease the variability in the performance of the pads in producing a given surface finish, the techniques do not serve to improve the overall quality of the surface finish.
As is evident from the aforementioned discussion, the CMP technique, while a preferred method, still has a number of difficulties in the practical application of the technique that need to be overcome. It is therefore an object of the present invention to provide a method and an apparatus for chemical mechanical polishing of the surface layer of a semiconductor wafer that overcomes many of the difficulties of current process to provide a more generally effective CMP process.